Fuel cells have been used as a power source in many applications, for example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell, and oxygen is supplied as the oxidant to the cathode. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued Dec. 21, 1993 and May 31, 1994, respectively, and commonly assigned to General Motors Corporation. MEAs include a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and a cathode catalyst on the opposite face.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack. Each cell within the stack comprises a membrane electrode assembly which provides its increment of voltage. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113 assigned to General Motors Corporation.
The electrically conductive elements sandwiching the MEAs may contain an array of channels or grooves in the faces thereof for distributing the fuel cells gaseous reactants over the surfaces of the respective cathode and anode. In the fuel cell stack, a plurality of cells are stacked together in electrical series while being separated one from the next by a gas impermeable, electrically conductive bipolar plate. The bipolar plate of a fuel cell stack has a practical volumetric flow turndown capability of about 10:1. This limitation can be partially addressed by reducing inlet pressure and/or increasing the stoichiometric ratio as stack throughput decreases. With a pressure turndown of about 3 and a low-flow anode stoichiometry of 4, one can realize a stack turndown ratio of about 120:1. Unfortunately, higher inlet pressure (at high throughputs) and/or higher reactant stoichiometries (at low throughputs) require more parasitic losses to be incurred, reducing system efficiency. Distribution of the reactants to each and every cell in the stack is accomplished by providing some pressure drop between the inlet and outlet manifolds. This pressure drop causes the flow to distribute more uniformly across all of the cells.
Therefore, it is desirable to provide a system that maintains a reasonable pressure drop while providing sufficient velocities and reactant concentrations at each cell for varying throughputs.